A conventional method of manufacturing an insulated electric wire is disclosed (see prior art document) wherein the insulated electric wire covered by an insulating coat has a rectangular cross-sectional shape. In the prior art, a conductor having a circular cross-sectional shape is rolled by a pair of rolls constituting a cassette roll die CRD to form the rectangular cross-sectional shape. Then, the conductor having the rectangular cross-sectional shape is annealed in an annealing furnace so as to remove distortion formed therein upon rolling to become flexible. Next, enamel varnish is covered on the conductor after annealing, and the enamel varnish covered on the conductor is baked in a baking furnace. The prior art document is Japanese Patent Publication No. 3604337.
In the conventional method of manufacturing the insulated electric wire, free rolls rotating freely without a drive mechanism are adopted as the pair of rolls. A distance between the rolls is adjusted small such that the conductor can be rolled in a width direction thereof, while passing through the distance between the rolls (see the prior art document).
For example, a reduction rate by one pair of free rolls is desirably in a range between 5% and 30%. When the reduction rate by the free rolls exceeds a predetermined value, the conductor is not rolled in the width direction even upon the rolling through the free rolls, and rolled in the lengthwise direction thereof.
That is, when the reduction rate by the free rolls is over the predetermined value, an angle of the conductor rolled with the rolls becomes large, thereby increasing a back tension applied to the conductor.
Therefore, when the reduction rate by the free rolls is large, a force exceeding a breaking load is applied to the conductor so that the conductor may be broken out upon the rolling operation.
As a result, according to the conventional method, only the conductor having a rectangular cross-sectional shape and a ratio of a thickness to a width of about 1:2 is manufactured.
In the conventional method, a driving roll is employed and the rolling rolls rotate at a predetermined speed, it is difficult to stably manufacture the conductor having a desired width due to facility-associated factors.
Further, in order to improve product quality and extend a length of the insulated electric wire, it is necessary to perform an entire process from the rolling where the conductor having the circular cross-sectional shape is rolled to have the rectangular cross-sectional shape to the enamel varnish-coating and baking process in a tandem arrangement. However, it is difficult to conduct the manufacturing process in the tandem arrangement because of width instability of the conductor.